The invention relates to security devices, methods for constructing such devices, and methods and apparatus for authenticating the devices.
Security devices such as holograms and diffraction gratings have become well known for protecting identification articles such as credit cards and the like.
A typical example is described in U.S. Pat. No. 4,269,473 in which a hologram is incorporated into a layer of an identification card. In this case the hologram includes machine readable characters as well as a visual representation of the other features of the card.
U.S. Pat. No. 4,184,700 describes the provision of a relief structure on a thermoplastic coating of an identification card or document which responds to incident, visible light to generate an interference pattern.
U.S. Pat. No. 4,544,266 describes a special diffraction pattern which is provided on an identity card or the like, the pattern comprising for example a hologram or diffraction grating. The diffraction pattern will diffract light at different wavelengths in different directions and this is used to provide an indication of whether a security device under test is authentic.
U.S. Pat. No. 3,542,448 discloses the recording of a number of holograms in different sub-areas of a storage medium so that upon exposure to light, coded information within the holograms can be determined.
GB-A-2016775 describes the provision of two optical markings on a substrate which cause a reading light be to deflected in different directions.
U.S. Pat. No. 4,140,373 describes a composite hologram which generates two machine readable codes which can be read at respective, different wavelengths. This suffers from the disadvantage that the existence of machine readable information is readily apparent and thus is likely to be fraudulently copied.